Technical Field
The present disclosure relates to planar transformers. More particularly, the present disclosure relates to planar transformers configured to reduce termination losses.
Description of Related Art
Transformers are used in many applications to transfer electrical energy from one circuit to another through inductively coupled conductors or windings. Typical transformers used in electronic devices include a ferromagnetic core around which copper wire is wound to form the primary and secondary windings. In some applications, however, such as in portable electronic devices, typical transformers are too large and heavy. Therefore, many of these electronic devices incorporate planar transformers, which are compact and light weight.
Planar transformers typically include a plurality of stacked circuit layers or PCBs, each of which has an electrically conductive trace that forms one or more transformer windings. The one or more transformer windings on each PCB include a first termination portion through which current enters and a second termination portion through which current exits. When the first and second termination portions are disposed close to one another and are separated by a narrow gap, they may be susceptible to the proximity effect. The proximity effect causes currents flowing through the closely-spaced first and second termination portions to crowd together and flow only in the narrow portions of the first and second termination portions that are closest to the gap. This narrow portion may extend approximately one “skin-depth” from the edges of the first and second termination portions that are disposed adjacent to the gap. As a result, the planar transformer experiences significant power losses commonly referred to as termination losses.
When a power supply providing power to the planar transformer switches at a high frequency, e.g., in the order of 100s of kHz, the “skin depth” of the first and second termination portions may be only a few thousandths of an inch. The resulting termination loss is often the primary cause of conduction loss in planar transformers, often accounting for about 75% of total alternating current (AC) resistance.